1. Field of the Invention
The present invention relates to satellite navigation systems, and in particular to worldwide reference stations for compiling lists of geostationary satellite based augmentation system (SBAS) satellites that will be visible to particular ground locations so that navigation receivers provided with this information from a network server can intelligently search for, acquire, and track the pseudorandom number (PRN) signals of the SBAS satellites visible to them.
2. Description of the Prior Art
The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to supplement the Global Positioning System (GPS). The goal is to improve the accuracy, integrity, and availability of GPS. WAAS was planned to let aircraft rely on GPS for precision approaches, and other phases of flight. WAAS uses a network of accurately surveyed ground-based reference stations in North America and Hawaii to detect errors in the GPS satellites' transmissions at the earth's surface in a variety of regions. The measurements are forwarded region-by-region to master stations. A Deviation Correction (DC) is sent in correction messages to geostationary WAAS satellites at least every five seconds. The geostationary WAAS satellites then broadcast correction messages back to North America and Hawaii so WAAS-enabled GPS receivers can include the corrections in their navigation position calculations. WAAS does not provide service to Europe or Asia because the geostationary WAAS satellites are not visible there, and the measurements that are taken are not applicable there.
The International Civil Aviation Organization (ICAO) refers to WAAS as a Satellite Based Augmentation System (SBAS). Europe and Asia have corresponding SBAS deployments, namely the European Geostationary Navigation Overlay Service (EGNOS), the Indian GPS Aided Geo Augmented Navigation (GAGAN), and the Japanese Multi-functional Satellite Augmentation System (MSAS). Commercial SBAS operations include StarFire and OmniSTAR. These have geostationary satellites parked above their respective service areas. So no one place on earth has visibility to WAAS, EGNOS, GAGAN, MSAS, and the others simultaneously.
WAAS position accuracies are supposed to be better than 7.6 meters (25′) for both lateral and vertical measurements, at least 95% of the time. Tests have shown it typically provides solution accuracies better than one meter (3′3″) lateral, and 1.5 meters (4′11″) vertical throughout most of North America. WAAS therefore meets required Category I precision approach accuracies of sixteen meters (52′) lateral, and four meters (13.1′) vertical.
Maintaining the integrity of the navigation system requires timely warnings when misleading data is being provided that could create hazards. The WAAS specification requires the system monitor and detect errors in the GPS and WAAS networks and notify users within 6.2 seconds. Certifying that WAAS is safe for instrument flight rules (IFR) requires proof that any error exceeding the requirements for accuracy will not go undetected. Specifically, the probability is stated as 1×10−7, and is equivalent to no more than three seconds of bad data per year. This provides integrity information equivalent to or better than Receiver Autonomous Integrity Monitoring (RAIM).
The WAAS ground segment includes several Wide-area Reference Stations (WRS) with precisely surveyed locations that monitor and collect information on the GPS signals, then send their data to three Wide-area Master Stations (WMS) using a ground based communications networks. The reference stations also monitor signals from the WAAS geostationary satellites, providing integrity information regarding them as well. In Oct. 2007 there were thirty eight WRS. Twenty of these in the contiguous United States (CONUS), seven in Alaska, one in Hawaii, one in Puerto Rico, five in Mexico, and four in Canada. Data from the WRS sites is used by each WMS to generate fast and slow sets of corrections. The fast corrections are for rapidly changing errors, and primarily concern the instantaneous positions and clock errors of the GPS satellites. These corrections are user position-independent, they can be applied immediately by receivers in the WAAS broadcast area. The slow corrections relate to long-term ephemeris and clock error estimates, and ionospheric delay information. Delay corrections are provided for points organized in a grid pattern across the WAAS service area. Each WMS sends the correction messages through redundant Ground Uplink Stations (GUS) to the geostationary satellites in the space segment, and rebroadcast to the user segment.
The WAAS space segment has many geosynchronous communication satellites that broadcast the correction messages from the Wide-area Master Stations for reception by the User segment. They also each broadcast range information like GPS satellites, effectively increasing the number of satellites available for a position fix. Originally, the space segment had two commercial satellites, Pacific Ocean Region (POR) and Atlantic Ocean Region-West (AOR-W). These were actually space leased on the Inmarsat III satellites which ceased WAAS transmissions on Jul. 31, 2007. Two new satellites, Galaxy 15 and Anik F1R, were launched in late 2005. Galaxy 15 is a PanAmSat, and Anik F1R is a Telesat. These too are leased services under the FAA's Geostationary Satellite Communications Control Segment contract with Lockheed Martin for WAAS geostationary satellite leased services. Lockheed Martin is contracted to provide up to three satellites through 2016. Ranging data from Galaxy 15 and Anik F1R has been flagged as “Precision Approach.”
SatelliteNMEAPRNLocationInmarsat 4F3NMEA #46PRN #13398°WGalaxy 15NMEA #48PRN #135133°WAnik F1RNMEA #51PRN #138107.3°W
A first order of business for a SBAS receiver is to collect the PRN mask. It needs this before it can make use of the SBAS messages. Collecting the PRN mask can take up to a minute. After receiving the PRN mask, the SBAS receiver must settle on the appropriate satellite health and differential GPS corrections to use. The ephemerides are needed to get the ranges and include the SBAS satellite in a fix. But, getting the ephemerides is a lower priority than getting the satellite health and differential GPS corrections. So, an autonomous SBAS receiver will generally not have SBAS data available before a first fix is possible. That first fix may not be very accurate, at least not up to what is usual for an SBAS assisted navigation receiver.
Galileo is a global navigation satellite system (GNSS) being built by the European Union (EU) and its European Space Agency (ESA). It will be redundant to the United States Global Positioning System (GPS) and the comparable Russian system, GLONASS. Galileo is expected to be operational by 2014 with two ground operations centers, one near Munich, Germany, and the other just east of Rome, Italy. Galileo is projected to make available more precise measurements than are widely accessible with GPS or GLONASS, especially at high latitudes. The political aim is to provide an independent positioning system upon which the European nations can rely, since Russia and the USA both have the military option to switch on signal encryption. The basic Galileo services are to be free and open to everyone, but its high-accuracy capabilities will be restricted to EU military users and commercial subscribers.